1. Field of the Invention
This invention relates to a method and apparatus for controlling the fuel flow in fuel cell systems.
2. Description of the Prior Art
A fuel cell is a device which directly converts chemical energy into electrical energy. A fuel, typically hydrogen, and an oxidant, typically air, are supplied to electrodes which are spaced apart by an electrolyte containing member. Electrons flow through an external circuit from one electrode to the other and constitute the electrical output of the cell.
Fuel and air must be supplied to the respective electrodes so that current can be continually supplied to a load in the external circuit. Often the fuel is steam reformed to produce hydrogen in a package outside the fuel cell called a reformer.
Fuel cell control systems have maintained the desired cell output by maintaining the operating temperature of the fuel cell since the cell performance is a function of the temperature. It is also known to monitor reactant pressure, humidity levels, electrolyte concentration, flow rates and a host of other parameters to keep the system operating under optimum conditions.
A more recent method for maintaining the desired cell output is described in U.S. Pat. No. 3,585,078, "Method of Reformer Fuel Flow Control" by R. A. Sederquist and John W. Lane, of common assignee with the present invention. A companion patent, 3,585,077, "Reformer Fuel Flow Control" by E. I. Waldman (applicant for the present invention) of common assignee with the present invention, describes and claims apparatus suitable for use with the method of the aforementioned patent 3,585,078. In those patents feed flow to the reformer is regulated as a function of the fuel cell gross current in combination with biasing the reformer feed flow as a function of reactor temperature. In one aspect of those inventions steam is provided as a primary flow to a variable area ejector wherein a gaseous fuel supply is the secondary flow. Steam flow is regulated depending upon the gross current and reactor temperature. As steam flows into the ejector it creates a low pressure area which draws or aspirates fuel into the ejector; the greater the steam flow, the greater the fuel flow. The fuel mixes with the steam and is conveyed to a catalytic reformer where this feed is steam reformed. The reformed mixture may be fed directly from the steam reformer to the fuel cell or into additional reforming equipment and then to the fuel cell.
A problem with the foregoing system is that the reformer experiences clogging from carbon build-up resulting in a continuously increasing ejector back pressure with time. Of course, the ejector is pumping fuel against this back pressure. The increase in back pressure due to clogging of the reformer prevents the proper fuel flow resulting in reduced efficiency of the system.
In systems where hydrodesulfurizing of the fuel is necessary such as in those systems described in U.S. Pat. No. 3,480,417 to H. J. Setzer, also of common assignee with the present invention, the ejector must also pump recycled hydrogen as well as the fresh fuel flow. The additional pumping required of the ejector, coupled with a further increase in ejector back pressure due to the additional components in the system, make it very difficult if not impossible to provide the required fuel flow even with no clogging of the steam reformer.